A scanning electron microscope (SEM) scans a primary beam of electrons along a sample surface and detects the secondary electrons that are emitted. An image is formed, with the intensity at each image point being proportional to the number of secondary electrons detected at the corresponding point on the sample. Most electron microscopes operate in a high vacuum to prevent scattering of the primary electron beam. Such electron microscopes typically use a secondary electron detector called an Everhard-Thornley detector, which includes a scintillator that emits light when struck by a secondary electron and a photomultiplier tube that amplifies the light to produce an electrical output signal. A photo multiplier tube typically has a gain of about 106 that is, for each electron that enters the detector, about one million electrons are generated for detection. Such SEMs cannot observe moist samples, such as biological tissue, because the evaporating moisture scatters the primary beam and the vacuum dries the sample.
One type of SEM, referred to as “High Pressure Scanning Electron Microscopes” (HPSEM), such as the ESEM® Electron Microscopes from FEI Company, the assignee of the present invention, has been developed for observing moist samples at relatively high pressure. HPSEMs are also useful for observing non-conductive samples, because ionized gas molecules serve to neutralize the sample. FIG. 1 shows an HPSEM 100 similar to the one described in U.S. Pat. No. 4,785,182 to Mancuso, et al. HPSEM 100 includes an objective lens 102 to which is attached a pressure limiting aperture 104 that allows electrons to move from the upper column to a sample 106, but restricts the flow of gas into the evacuated electron column. The pressure limiting aperture 104 allows the pressure in the sample chamber to be significantly higher than the pressure in the electron beam column above aperture 104, so that electrons are not scattered by gas molecules along most of their path.
A positive voltage relative to sample 106 is applied to a detector 110, which consists of an electrode that is concentric with the optical axis. Secondary particles emitted from the sample 106 are accelerated toward the detector 110 and collide with gas molecules, producing additional charged particles, which in turn collide with other gas molecules to produce even more charged particles. Such a process is called a “cascade.” The ultimate number of charged particles produced in this manner is proportional to the number of secondary particles emitted at the substrate, thereby producing an amplified signal corresponding to the number of secondary particles. The electron source and much of the path of the primary beam is maintained in a high vacuum by the aperture 104 that passes the primary beam but prevents most gas molecules from entering the column. Gas pressure at the sample in an HPSEM is typically maintained at around 0.1 to 50 Torr, and more typically between 0.5 and 5 Torr.
The amplification of the secondary electron signal in an HPSEM depends on the gas pressure, the electron path length, and the voltage between the sample and the detector. The amplification is typically much lower that that of an ET detector. Higher gas pressure allows for more collision and may better preserve some types of samples, such as hydrated biomaterials, but too high a pressure impedes the gas cascade and reduces the amplified imaging signal. A longer path length generally results in more collisions. Magnetic and electric fields can be used to increase the path length of the secondary electrons to provide greater amplification. For example, U.S. Pat. No. 6,972,412 for “Particle-Optical Device and Detection Means” to Scholtz et al., assigned to the assignee of the present invention, describes using magnetic and electric fields between the detector and the specimen holder to lengthen the path of the secondary electrons to produce increased amplification. Increasing the voltage between the sample and the detector provides more energy to the electrons to ionize gas molecules. Too high a voltage, however, causes dielectric breakdown of the gas, that is, a self-sustaining gas ionization cascade. The signal is then no longer proportional to the secondary electron current produced by the primary beam and is no longer useful for forming an image of the sample.
Many detectors for HPSEM's use a circular electrode that is concentric with the optical axis as shown in FIG. 1. Some systems use an off-axis detector. For example, U.S. Pat. No. 7,193,122 of Jacka, et al. uses an off axis detector chamber that is maintained at a lower pressure than the sample chamber. A grid positioned in front of the detector entrance attracts electrons, which pass through a pressure limiting grid to enter a differentially pumped chamber. Because the chamber interior is maintained at a lower pressure than the sample chamber, the high voltage required by a scintillator detector does not cause breakdown of the gas.
M. R. Phillips and S. W. Morgan, in “Enhanced High Speed SE Imaging in a VPSEM Using a Frisch Grid,” Micros Microanal 12 (Supp 2) 2006 describe the use of a Frisch grid near the anode of a detector to shield the anode from current induced by ion movement beyond the grid, so that the anode signal primarily reflects the electron motion, which is faster and therefore increases the detector bandwidth. While Phillips et al. increase the detector bandwidth, they do not address the detector gain or noise.
Thus, the amplification of the secondary electron signal in an HPSEM is limited by a number of factors. It would be desirable to increase the amplification to improve the sensitivity of the microscope.